Carbon nanotubes (CNT) have potential uses in a wide array of applications. In particular, regularly spaced arrays of CNTs are expected to have wide ranging applications, either as new and novel devices or as substantial improvements over existing technology. Carbon nanotubes are electrically conductive, have high strength and stiffness, and exhibit electromechanical coupling similar to piezo-electric materials: mechanical deformations of a CNT can induce charge transfer into the nanotube, and injected charge can produce deformation or mechanical stress in the nanotube.
These properties combine to give CNTs their versatility in a wide range of uses. One example uses an array of nanotubes as a filter medium in electrophoretic separations of biomolecules. Electrophoretic separation occurs through the differential transport of polyelectrolytes (DNA, proteins, etc.) through a porous medium in the presence of an electric field. The medium acts as a sieve, producing a size-dependent mobility of the molecules. Current state-of-the-art biomolecular analytical systems use polymer gels as sieves. Such gels typically have a wide, random distribution of pore sizes, often extending into size ranges much greater than that of the molecules of interest (1–10 nm), which limits the efficiency of such gels in separations. In addition, polymer gels are susceptible to radiation damage and thermal decomposition, limiting their utility in analysis in extraterrestrial environments of interest to NASA. A regularly-spaced array of carbon nanotubes, with well-defined diameters and tube-tube separations in the range of 1–10 nm, is expected to be much more efficient at such separations, much more amenable to miniaturization, and much more tolerant of harsh environments owing to the high stability of CNT. Arrays of CNT would thus advance this technology and make these methods useful in many new environments of interest to NASA.
Another set of applications for CNT arrays takes advantage of their mechanical and electrical properties. The high strength and stiffness of these structures makes free-standing CNT excellent candidates for high-Q, high-frequency, nano-scale oscillators. In addition, the electromechanical coupling provides a means by which to either excite vibrations (by applying an RF signal), or to monitor the motions/vibrations of nanotubes by monitoring the current induced as they mechanically oscillate. Arrays of CNT have thus been proposed for use as nanometer-scale RF filters, RF signal detectors and analyzers, and spectrum analyzers for mechanical vibrations (the “electronic ear”). In addition, nanotubes can be functionalized with various chemical substituents to make various molecules bind to them more or less strongly. A nanotube so functionalized would be expected to change its resonant frequency upon the attachment of a target molecule, so that arrays of functionalized nanotubes could also be used as nano-scale chemical sensors (the “electronic nose”).
Finally, the small size and high aspect ratio of CNTs suggests that they will give off electrons by field emission at much lower voltages than the (much larger) metal field emitter tips currently in use. A regular array of independentlyaddressable nanotubes could thus be used as a low-voltage, low-power array of field emitters, for field emitter displays.
However, all of these applications rely on the ability to produce arrays of nanotubes with regular, well-defined inter-tube spacings. The nanotubes would also need to have their aspect ratios (diameter and wall-thickness vs. length) carefully controlled, to ensure that they stood upright on the substrate, rather than bending over. Many previous studies have reported mats of CNT catalytically grown on smooth substrates. These nanotubes are produced by the decomposition of carbon-containing gases (e.g., C2H4, CO) over nanometer-sized particles of catalytic metals (e.g., Fe, Ni). The nanotubes nucleate upon the metal particles and grow longer as carbon is produced by decomposition of the source gases upon the catalyst. In such studies, catalyst particles are produced by deposition of catalytic metals onto the substrate, either physically or chemically, so that the resultant particles are randomly distributed on the surface and have a wide distribution of sizes. Since the size of growing nanotubes is determined by the size of the catalyst particle from which it nucleates, this process results in growth of nanotubes with a wide diameter distribution, positioned at random upon the substrate surface.
To date, most demonstrations of nanotube growth from such supported catalysts have yielded densely-packed “fields” of nanotubes: neighboring nanotubes are typically in contact with one another, and the size distribution is wide and not well controlled. Clearly, such an “array” does not have the properties needed for the applications discussed above: nanotubes in contact cannot move or be addressed independently, and a wide range of diameters implies a wide range of resonant frequencies, making spectral analysis as described above impossible.
“Bulk” methods for depositing catalytic metals, such as physical or chemical vapor deposition, will result in a stochastic distribution of catalytic particle sizes and locations and thus is not suitable for generating ordered arrays. Lithographic methods, such as photolithography or electron-beam lithography, could work in principle, however photolithography does not have the resolution to make nanometer-sized particles, while conventional e-beam lithography will in general be too slow for large-scale throughput and production of devices using these CNT arrays.
However, recently a technique has been developed for producing geometrically regular self-assembled nanotube arrays with excellent uniformity. This process is based upon the self-organizing formation of highly uniform pore arrays in anodized aluminum films. First, a nanochannel alumina structure is formed by anodizing an aluminum film under conditions that lead to hexagonally-ordered arrays of narrow channels with very high aspect ratios. The nanochannel alumina structure is then used as a template for the growth of nanotube arrays of carbon and other materials, including metals and some semiconductors. The full process technique is disclosed in Appl. Phys. Lett., vol. 75, pg 367 (1999), and is incorporated herein by reference. Utilizing this technique the authors were able to grow nanotube arrays comprising uniform carbon nanotubes with a diameter of 32 nm. Despite the promise of this new technique, it is a complex procedure, and requires thick (>10 μm) Al films as the starting material, limiting its usefulness in some applications.
Accordingly, a need exists for improved method of producing, from catalytic metal particles supported on substrates, an array of nanotubes that are regularly spaced (in particular, they must not be in contact), and which have the same diameter and length (or at least have a very narrow distribution of sizes) in order that they have nearly identical electrical and mechanical properties.